Electrical relay devices which operate using electromagnetic principles are a well known and popularly used component employed in many electrical circuit applications. The relay device of the present invention is of the DC contactor type. These relay devices may be operated under high voltage/high current conditions typically having voltages in the 270 Volt DC range. One of the major consequences for relays that operate at these high voltages is that they normally operate in a "hot switching" (switching under load, causing arcing) environment with normal operating currents ranging from 25-1000 amps. The relays also have been known to have an overload interrupt capacity of 100 to 2500 amps and have also been known to have the capability to maintain low contact resistances on the order of 5.0-0.1 milliohms.
Relays of the DC contactor type can experience problems in "hot switching" environments in that there is no current zero point in the DC signal (as opposed to that of an AC signal) which can aid in breaking the arc which results from separation of the relay contacts while current is passing through them. Arcing due to contact "bounce" or "make" may cause puddling (contact melting) and possibly the welding together of the relay contacts which is the joining of the contacts together. It is difficult to extinguish these arcs which usually occur during the connection, or making, or the disconnection, or breaking, of the contact surfaces.
Arcing in relays results from the following phenomenon. The contacts may start off in the closed circuit "make" or open circuit "break" position. As they begin to come together or as they begin to separate from one another the separation between contact surfaces is infinitesimal. Hence, the electric field strength is intense and electrons are accelerated across the gap between the contacts. This leads to an electron avalanche effect resulting in the ionization of particles in the gap. Even if the relay contacts are maintained in a vacuum chamber, arcing may still occur in the absence of air.
In the cases of both an air-filled or an evacuated (vacuum) environment, continuous arcing may commence and a great amount of heat may be generated which melts the contact material. The hot, easily ionized material forms a contact plasma (plasma) as the contacts continue to come together, or as they separate. An arc column will then begin to form. This arc column will form from contact plasma in the case of a vacuum environment or from contact plasma along with ionized particles in the case of an air-filled environment. Contact material plasma and/or ionized particles will build up and develop a continuous trail of charged particles between the contacts and thereafter an arc will occur. The arc will finally be extinguished when the contacts come together, or when the contacts fully separate because the electric field strength between the contacts is not high enough to ionize contact material electrons.
When arcing occurs, a phenomenon known as puddling may occur which describes the actual melting of the contacts surface material. Puddling may cause craters to form on the contact surfaces in those locations where contact material has been melted away or when melted contact material has hardened in a coarse manner. Puddling may further lead to the welding together of the contacts making it difficult to separate them.
Welding refers to the joining of the contacts together either microscopically or more grossly due to the hardening of the melted contact material between the contacts. The occurrence of arcing and its associated puddling or welding of the contacts are most undesirable as they lead to deterioration of the relay contacts, dielectric breakdown, and finally, relay failure.
As from the differences already noted between DC contact rely "hot switching" in a vacuum, versus that in air, the following is also to be noted regarding relay "hot switching" in a vacuum. The vacuum has 1) a much greater voltage standoff capability, and 2) significantly reduces plasma formation. Such a reduction in plasma formation is approximately eight orders of magnitude less than the corresponding formation of ionized particles in air-filled chambers. The vacuum also eliminates contaminants which cause increased contact resistance over the operating life of the relay, eliminates ionized particles which cause oxidation and increased contact resistance, protects against explosions in hazardous environments, and petits the use of hard contact materials without sacrificing low contact resistance. By reducing contact wear, relay life will be increased.
In order to successfully connect relay contacts under load in either a vacuum or in an air-filled environment it is a common occurrence for the contacts to "bounce" during the period of contact closure. It is important at this juncture to note that the making of an electrical connection by connecting two contacts to one another is referred to as contact make or "make" while the disconnecting or separating of these contacts is referred to as contact break or "break".
It is necessary to reduce any arcing, puddling, and/or welding between the contact materials so as to enable the relay contacts to completely be disconnected from each other every time a contact "break" is desired.
In the DC contactor relay design of the present invention, the creation and/or occurrence of ionized particles or contact plasma may be reduced by the elimination of air such as by employing a vacuum chamber so as to minimize particle ionization, and by utilizing contacts made of a high temperature material which is hard to ionize. It is also desirable to increase the contact gap quickly upon contact break so as to allow the gap to increase before a sufficient amount of contact plasma and/or ionized particles, which are needed to sustain an arc, form in the gap. It is important to note that a vacuum also reduces the gap distance required to reach open circuit voltage.
It is also desirable to use additional means to increase the voltage required to sustain an arc. This may be accomplished by using permanent magnets to alter the field between the contacts, thereby making it more difficult for the arc sustaining ionized particles or contact plasma to be maintained. Therefore, the arc will be extinguished. Arc chutes which are well known in the art, and which draw the arc away from its straight path between the contacts, may also be employed to augment this function.
The employment of vacuum technology in relay design also reduces design conflicts and improves relay performance in that large contact cross sectional areas are no longer required to maintain low contact resistance. This results in a lower contact resistance per unit area and, therefore, reduced relay size and weight. Further, large contact gaps are not required in a vacuum environment as the vacuum is a far better dielectric than air. This feature also facilitates a reduced relay size.
The use of a vacuum relay device also provides for a faster acting actuator as there is no air drag on the moving contact. Further, a more efficient armature design may be accomplished in the absence of air. These above-mentioned factors also lead to a reduction in both the size and weight of the relay device. The vacuum also facilitates fast arc dissipation as the arcs move 100 times faster in a vacuum than in air. This feature also facilitates a size reduction.
The relay device of the present invention is capable of interrupting high current values at 270 VDC. In order to do so, conflicting design criteria come into play. The relay requires a large contact gap which, in turn, tends to increase the physical size and weight of the relay. Such a relay also requires quick retracting contacts which requires a corresponding decrease in the weight of the contacts.
In the area of reducing power consumption by these relays, it is desirable to minimize the contact resistance. This requires a large contact cross-sectional area which tends to increase contact size and weight and requires a corresponding increase in coil size and weight. The minimization of contact resistance also requires a large contact force which requires an increased coil size and weight. Power consumption could also be reduced by minimizing coil heating. This requires a small actuator coil which decreases the size and weight of the coil. Power consumption may further be reduced by allowing puddling to occur. This requires a large actuator force upon the contacts, and therefore, increases the coil size and weight. Lastly, power consumption may be reduced by using smaller parts which allow for the decrease of the size and weight of the relay device and its components.
Relays are basically comprised of a coil which is energized by an electrical current flowing therein. The current flowing therein creates an electromagnetic field which moves an armature in such a manner so as to bring at least two electrical conductors or contacts into connection with one other. As a result, the electrical circuit to be serviced by the conductors is closed and current will flow through the desired circuit. It is at the location of these contacts or conductors where the aforementioned arcing and its associated problems occur.
Arcing is more severe in DC relays than in AC relays. This is due to the fact that the AC signal varies sinusoidally and periodically over time and through a zero value at which point a circuit disconnect or "break" may be effected. The effects of the arcing, puddling, and welding, while they may not be totally eliminated, can be reduced by a proper design concept. One way to eliminate or alleviate the problems associated with arcing, puddling, or welding is to provide for a significant amount of force during that instance in time when it is desired to disconnect or separate ("break") the connection between the contacts. This application of force to effect a contact break is known in the art as "impact break". The present invention utilizes an armature shaft in motion prior to the contact break in order to perform this "impact break".
Relays of the DC contactor type which utilize "impact break" methods, come in a number of varieties. The method employed in the present invention utilizes the kinetic energy of a moving armature to provide the physical force necessary to "break" the connection between the movable contact and the stationary contact of the relay device. This is accomplished by using a sudden force of impact which will disconnect the connection between the contacts and break any welding connection which may exist between them.
The present invention is a new and improved version of a "linear" impact break relay. An armature and plunger, upon the excitation of a coil and subsequent magnetic field established thereby, is driven in such a direction (linear direction) towards the relay's electrical stationary contacts. The driving force is typically the magnetic flux linking a stator/armature assembly, and the resultant force moves the armature towards the stator, which activates movement of a plunger attached to the armature. The armature or plunger typically drives a conductor or moving contact in the same (linear) direction as its own movement until the conductor or moving contact makes contact with one or more stationary contacts in order to complete the electrical circuit to be serviced by the relay. Upon this contact "make" the electrical circuit is now complete and operational.
When the coil is de-energized, the armature or plunger will be driven in the opposite direction, usually by the force of a biased spring, thereby forcing the moving contact driven by it away from the stationary contact thereby "breaking" the connection between the contacts and opening the electrical circuit.
The force of the returning armature is applied in line with, or linear to, the contacts so as to effect a contact "impact break" with a force which is also in line with or linear to the motion of the armature.